S. Perotto

Impact of Biocontrol Pseudomonas fluorescens CHA0 and a Genetically Modified Derivative on the Diversity of Culturable Fungi in the Cucumber Rhizosphere

ABSTRACT Little is known about the effects of Pseudomonasbiocontrol inoculants on nontarget rhizosphere fungi. This issue was addressed using the biocontrol agent Pseudomonas fluorescens CHA0-Rif, which produces the antimicrobial polyketides 2,4-diacetylphloroglucinol (Phl) and pyoluteorin (Plt) and protects cucumber from several fungal pathogens, includingPythium spp., as well as the genetically modified derivative CHA0-Rif(pME3424). Strain CHA0-Rif(pME3424) overproduces Phl and Plt and displays improved biocontrol efficacy compared with CHA0-Rif. Cucumber was grown repeatedly in the same soil, which was left uninoculated, was inoculated with CHA0-Rif or CHA0-Rif(pME3424), or was treated with the fungicide metalaxyl (Ridomil). Treatments were applied to soil at the start of each 32-day-long cucumber growth cycle, and their effects on the diversity of the rhizosphere populations of culturable fungi were assessed at the end of the first and fifth cycles. Over 11,000 colonies were studied and assigned to 105 fungal species (plus several sterile morphotypes). The most frequently isolated fungal species (mainly belonging to the generaPaecilomyces, Phialocephala, Fusarium, Gliocladium, Penicillium, Mortierella, Verticillium, Trichoderma, Staphylotrichum, Coniothyrium, Cylindrocarpon, Myrothecium, and Monocillium) were common in the four treatments, and no fungal species was totally suppressed or found exclusively following one particular treatment. However, in each of the two growth cycles studied, significant differences were found between treatments (e.g., between the control and the other treatments and/or between the two inoculation treatments) using discriminant analysis. Despite these differences in the composition and/or relative abundance of species in the fungal community, treatments had no effect on species diversity indices, and species abundance distributions fit the truncated lognormal function in most cases. In addition, the impact of treatments at the 32-day mark of either growth cycle was smaller than the effect of growing cucumber repeatedly in the same soil.

Extracellular polysaccharides are involved in the attachment of Azospirillum brasilense and Rhizobium leguminosarum to arbuscular mycorrhizal structures

Arbuscular mycorrhizal (AM) fungi, one of the most important component of the soil microbial community, establish physical interactions with naturally occurring and genetically modified bacterial biofertilizers and biopesticides, commonly referred to as plant growth-promoting rhizobacteria (PGPR). We have used a genetic approach to investigate the bacterial components possibly involved in the attachment of two PGPR (Azospirillum and Rhizobium) to AM roots and AM fungal structures. Mutants affected in extracellular polysaccharides (EPS) have been tested in in vitro adhesion assays and shown to be strongly impaired in the attachment to both types of surfaces as well as to quartz fibers. Anchoring of rhizobacteria to AM fungal structures may have special ecological and biotechnological significance because it may facilitate colonisation of new rhizospheres by the bacteria, and may be an essential trait for the development of mixed inocula.

Zinc ions alter morphology and chitin deposition in an ericoid fungus

A sterile mycelium PS IV, an ascomycete capable of establishing ericoid mycorrhizas, was used to investigate how zinc ions affect the cellular mechanisms of fungal growth. A significant reduction of the fungal biomass was observed in the presence of millimolar zinc concentrations; this mirrored conspicuous changes in hyphal morphology which led to apical swellings and increased branching in the subapical parts. Specific probes for fluorescence and electron microscopy localised chitin, the main cell wall polysaccharide, on the inner part of the fungal wall and on septa in control specimens. In Zn-treated mycelium, hyphal walls were thicker and a more intense chitin labelling was detected on the transverse walls. A quantitative assay showed a significant increase in the amount of chitin in metal-treated hyphae.

Morphological analysis of early contacts between pine roots and two ectomycorrhizal Suillus strains

Abstractu2002Selection of ectomycorrhizal strains for application in forestry is mostly based on the evaluation of symbiotic performance in small-scale experiments. Two Suillus collinitus strains isolated from a Mediterranean and an alpine area were inoculated onto two pine tree species (Pinus pinea and P. nigra ssp. laricio var. corsicana) typical of these two environments. The early events during contact between the cell surfaces of plant and fungal partners were analysed morphologically using ultrastructural and immunocytochemical techniques. All four plant-fungus combinations led to a similar degree of mycorrhizal infection and to a similar colonization pattern. The first contact of fungal hyphae with root cap cells usually involved breakdown of the outermost electron-opaque layer of the plant cell walls. Hyphae further developed between this layer and the underlying wall strata. Ultrastructural observations revealed that S. collinitus strain J3-15-24, isolated from a Mediterranean area, induced a defence reaction in the roots of P. nigra, which grows typically in alpine areas. These observations suggest functional differences between the two fungal strains in their mycorrhizal capabilities.

Arbuscular mycorrhizal fungi and soil bacteria: from cellular investigations to biotechnological perspectives

The rhizosphere is a dynamic environment in which bacteria, viruses, fungi, and microfauna, develop, interact and take advantage of organic matter released by the root (Weller and Thomashow 1994). A substantial consequence of this richness, in comparison with the bulk soil, is an intense microbial activity that results in changes in root development and growth of the whole plant. Two main groups of microorganisms are stimulated to grow in the rhizosphere: saprotrophs and symbionts, both comprised of bacteria and fungi that can be detrimental, neutral or beneficial to plants. In this article, we will focus on two categories of beneficial microorganisms of the rhizosphere, the plant-growthpromoting rhizobacteria (PGPRs) and the mycorrhizal fungi.

Arbuscular mycorrhizal fungi and their endobacteria

Morphological and molecular evidence indicate that AM fungi have been successful in time and space thanks to a long co-evolution with their host plants. In addition to this well known interaction, they also associate with bacteria that reside in the fungal cytoplasm. The presence of bacterial endosymbionts is not a sporadic event, at least in the Gigasporaceae, where they have been found in many species. This endosymbiosis between bacteria and AM fungi adds a further level of complexity to arbuscular mycorrhiza because the cyclical AM symbiosis, according to Margulis and Chapman (1998), harbors a probably more permanent symbiosis with endobacteria.